Aerodynamic particle sizing systems are used to measure the sizes of particles in a gas. The particles can be in the form of a powder, i.e. solid particles, or droplets of a liquid. According to a time of flight measurement technique, a gas containing particles to be measured is accelerated through a nozzle and is directed through two spaced-apart laser beams in a vacuum chamber. The laser beam are detected by photomultiplier detectors. Particles passing through the laser beams cause successive output pulses on the two photomultipliers. The time required for the particles to travel between the two laser beams is a known function of particle size. The output signals from the photomultipliers are analyzed to determine particle sizes. The results of the measurement can be tabulated in a computerized system to provide a particle size distribution. The sizing system can also count the particles being measured. The total volume of particles in a sample can be determined from the number of particles and the particle size distribution. Aerodynamic particle sizing systems are disclosed in U.S. Pat. No. 3,854,321 issued Dec. 17, 1974 to Dahneke; U.S. Pat. No. 4,895,034 issued Jan. 23, 1990 to Poole; U.S. Pat. No. 4,917,494 issued Apr. 7, 1990 to Poole et al; and U.S. Pat. No. 4,938,592 issued Jul. 3, 1990 to Poole et al.
Inhalers are widely used to administer medication in the form of an aerosol to patients. A typical inhaler includes a metered dose inhalator attached to one end of a generally cylindrical chamber. The other end of the chamber is placed over the nose of the patient. When a button on the metered dose inhalator is pushed, a predetermined dose of particles of the medication is sprayed into the chamber and is inhaled by the patient.
It is important to verify the accuracy of the medication dose delivered by the inhaler to the patient. The dose depends on a variety of factors, including the inhalation characteristics of the patient, the size and type of particles that form the medication, the propellant used for expelling the medication from the metered dose inhalator, the size and shape of the chamber, etc. It has heretofore been extremely difficult to assess the performance of such inhalers under a variety of conditions. While the particle sizing systems described above accurately measure particle sizes, such systems typically have a fixed flow rate that is quite different from the flow rates at which inhalers operate. Furthermore, inhalers are designed to operate at a variety of different volumes and flow rates depending on the application. Thus, existing particle sizing systems have not been suitable for directly assessing the performance of the inhalers described above.
It is a general object of the present invention to provide methods and apparatus for volumetric sampling of a particle-laden gas.
It is another object of the present invention to provide methods and apparatus wherein a predetermined volume of a gas is drawn into a sample chamber at a first flow rate and is delivered to a measurement instrument at a second flow rate.
It is a further object of the present invention to provide methods and apparatus for measuring the dosage of particles in a predetermined volume of a particle-laden gas.
It is yet another object of the present invention to provide methods and apparatus for accurately accessing the performance of inhalers used for delivering medication in the form of an aerosol.
It is a further object of the present invention to provide methods and apparatus for volumetric pollution control sampling of smokestacks and other exhaust stacks.